In circular sedimentation tanks or sludge thickeners the sludge solids, which settle to the bottom of the tank are normally moved from the periphery along the tank bottom to a sludge hopper at the centre of the tank by a rotating sludge collecting device which rotates around the centre of the tank. Collection of sludge is usually facilitated by inwardly sloped or conical bottom surfaces in such tanks.
The sludge collecting device generally consists of a large diameter central support tube which supports a gear drive unit and a centre bearing from which is suspended a drive cage structure extending down to just above the tank floor. Mounted on the drive cage are two opposed sludge collecting rake arms which extend outwardly to opposite walls of the circular tank. At the bottom of the rake arms are mounted a row of rakes or plow blades. These rakes or blades are usually mounted at 45 degrees to the direction of travel of the arms. When the device is rotating the plow blades will gradually rake the bottom sludge towards a centre hopper. The sludge is then removed by pumping for further treatment. The central drive support tube typically also serves as an inlet tube for fluid flow into the tank and is provided with inlet ports into the tank at or near the designed operating liquid level. Clarified liquid exits the tank over an effluent weir running continuously along the outer periphery of the tank. The flow that spills over the weir is collected in an effluent trough which runs all along the outside of the tank.
A significant portion of the solids entering the tank float and tend to collect at the liquid surface together with other liquid floating matter such as grease and oil. This layer which is a mixture of solids, oils and greases in varying proportions is generally known as scum.
In sludge thickening tanks using a dissolved air floatation method the surface layer is mainly sludge made buoyant by a large number of tiny air bubbles attached to the sludge particles. The air bubbles are produced by releasing high pressure air into the tank inlet pipe.
To improve the flow pattern inside the sedimentation tank and to prevent short circuiting along the tank surface from the inlet ports to the outlet weir, a cylindrical inlet baffle is suspended around the centre inlet area, extending about half way down from the liquid surface.
In circular sedimentation tanks the scum and floatables are expected to float outwardly towards the tank periphery. To prevent the scum from escaping over the outlet weir a continuous, shallow baffle plate is placed just inside the weir at the liquid level.
Typical scum collecting equipment consists of a travelling scum skimming blade and a scum weir and hopper located near the periphery of the tank. The scum skimming blade is supported from one of the sludge rake arms. As the mechanism rotates the surface skimming blade sweeps along a narrow band around the inner edge of the scum weir and pushes the scum into a scum hopper. The collected scum is then pumped away.
In many cases such conventional scum removal equipment works less than satisfactorily. This is particularly the case when the sewage is mixed with industrial wastes and greases that do not flow easily. Scum that remains on the surface for a long period will become odorous and result in odor complaints. Also the scum often gets trapped inside the cylindrical inlet baffle, where it can remain in a thick layer and again cause odors. Some prior designs have provided small scum ports in the inlet baffle at the liquid surface, but this is not always effective, as there is no positive action which drives the scum layer out of the baffle area. This is especially true for thick and cohesive scum, which may not be able to easily pass through a small port.
In cold climates there is an additional problem if scum is not removed regularly. Floating scum on the liquid surface may cool down and freeze solid. Such frozen material will further inhibit the removal and may also cause damage to the skimming mechanism or overloading of the gear drive.
What is desired is a scum skimming system that continuously cleans the entire circular tank surface and which prevents scum from building up and creating odor and icing problems. What is also desired is a scum removal system that prevents any unwanted loss of liquid from the tank by effectively separating all manner of scum from the clarified liquid. It is also desired to provide an improved scum skimming system which is completely compatible with commonly used sludge removal methods and which adapts easily to circular sedimentation tanks.
Further it is desired to provide a scum skimming system that is simple and economical, that can handle varying conditions and can be easily adjusted to give desired scum consistency for mixing and pumping without excess water. As such, it is preferable to have a system that uses readily available hardware, has few moving parts and that adapts t existing sludge collecting equipment.